The 986 uses a common ground for the speakers on the OEM HU, whilst most aftermarket HUs will use a floating ground. The "adapter" is a floating ground adapter and also degrades sound a bit.
The absolute best route would be to run new dedicated negative wires for each speaker from the new HU to the output neg wires at the amp (after its gone). The car has dedicated negative speaker wires from the amp to speakers, just not from HU to amp.
This is what I did.
Thanks Bruce!
